Not sure how your proposed bar would work. In theory you are putting 4 mountings (4 straps -2 from each belt) onto 2 mounting points. I would expect them to be strong enough, but it depends on the loads imposed.

From your pictures, would it be possible to fit a bar across the rear stays of the cage, near to the rear bulk head? You should be able to source suitable clamps. If it does not t need to be FIA approved,you could make your own. If the belt bar is behind the stays, the stays give extra support. The bar for the belts does not have to be straight. It can be angled to allow the ends to clamp onto the rear stays. Some historic stage rally Minis use this method.

Thanks,
I am not entirely sure what you mean ... (English is not my mother tongue). I am, also limited by lack of experience. I have never such a solution before and have difficulty imagining it .

A picture or a sketch would help.

I am also not sure where these clamps should be connected to the cage. At the base where the cage connects to the chassis in the rear , or to the main bar running along side roof pillars ( this way the bar would bun directly behind the seats.

Re: Strong enough.

I am assuming ( though as they say : "assumption is the mother of all f...ups " ) that the OEM seat bet mounting are strong enough for seat belt mounting. In this case using one seat belt per one OEM mounting bolt or two per two bolts or should make no difference. Also if you mounting two shoulder strips instead of one for the same weight should make no difference.

Picture : the yellow circles highlight stress areas:

belts xxx mounting .jpg (27.56 KiB) Viewed 2456 times

The effective load for mounting point will be exactly the same as the forces will simply spread between four shoulder straps and two mounting holes, in fact due to some elasticity of the belts and the bar itself the force acting directly on the bolts should be marginally less abruptly applied in case of a sudden impact. Only when the elasticity of the belts and the bar will be exhausted (few mili-seconds after impact) will the full impact force be applied to these OEM bolts And if the driver and co pilot are of different weight , the force acting on the two OEM bolts will be equalized will be equalized between the bolts

The load per OEM mounting will be the same as if applied by a single shoulder strap applied to a single OEM mounting. It follows that mounting I proposed has no impact on the safety ( given the same impact and passenger/driver weight).

Elegant though it is, there are some issues with this solution.
In case of an accident the load of both passengers will put a large bending moment on the bar. This bar has a limited stiffness (to be determined by size shape and material) and will bend.
The bending puts a force on the mounting points they were not set up for (straight pull) so they may collapse or shear off.
A bar with high stiffness may be strong enough to prevent this. Alternatively you may add a fixation in the middle, between the pairs of straps.

Since you have a roll bar anyway, I'd look into fitting a horizontal bar on that: either as direct fixation for the straps, or as guide for the straps (4 'rings' welded on?) with the straps fixed to some point near the car body; possibly the OE belt mounting points

And quite important: where on the body do you plan to fix the other ends of your belts? The forces there will be equally great of course.

your posts are so well written it is difficult to understand your native language is not English.

Perhaps a couple of diagrams will help. ( is it ok Guy? )

I am assuming your cage is similar to Diagram 1. This shows main hope (Red) , 2 rear stays (green) and the diaganol. (blue. )

If you insert a bar for seat belts it will be across the back stays as shown in Diagram 2. ( seatbelt bar in yellow)
This is ok, and often used but it means welding the bar in, because it interferes with the diagonal. ( At point A) So in effect the seatbelt bar is in 2 pieces and welded into the rollcage.

In your case you dont want to weld, so another option is to make the seatbelt bar with 2 bends in it. ( see diagram 3) A bar of this shape will then miss the diagonal . It can be welded or clamped to the rear stays. ( Sorry for the diagram. I had a picture of the bar installed in a Mini, but deleted it !)